1. Field of the Invention
The invention relates to the instruments that assist in the piloting of aircraft. More specifically, it relates to the onboard instruments of aircraft whose piloting requires, for technical reasons or for regulatory reasons, the presence of standby instruments to display essential navigation data in cases where the main display systems fail.
2. Description of the Related Art
In one typical example, for a commercial aircraft carrying passengers, the instrument panel of the aircraft comprises, among others:                a main readout for displaying, with great accuracy, a horizon, an attitude of the aircraft, and altitude, heading and speed data; this readout receives the information calculated by a computer of the aircraft from data received from various sensors;        an integrated electronics standby instrument (well-known in English flying literature) making it possible to display flight parameters such as a horizon, an altitude and a speed and, where appropriate, some other data independently of the main system, in a more summary manner and with lower accuracy. The information displayed is calculated directly by the integrated electronics instrument that displays on one and the same color screen all the standby information. The sensors associated with the integrated electronics standby instrument, such as pressure sensors for measuring total pressure Pt and static pressure Ps of the air surrounding the aircraft and accelerometers for determining the attitude of the aircraft are normally integrated in this instrument. In the event of failure of the primary display system, the pilot uses the data from the standby instruments.        
Currently, the instrument panels have only a single integrated electronics standby instrument, even when the aircraft is piloted by two pilots. The integrated electronics standby instrument is placed in the center of the instrument panel and can be used by both pilots. With the appearance of very large carrier aircraft, aircraft constructors have expressed a desire to place two integrated electronics standby instruments that can each be used by one of the two pilots.
Since each integrated electronics standby instrument comprises its own sensors, the flight parameters calculated by the two integrated electronics standby instruments are not strictly identical and their simultaneous display on both integrated electronics standby instruments could lead to confusion between the two pilots. To avoid this problem, just one of the integrated electronics standby instruments has been used to display the flight parameters, with the other integrated electronics standby instrument displaying navigation parameters such as the route to be followed and automatic piloting setpoints for the aircraft. Each integrated electronics standby instrument can be configured either to display the flight parameters or to display the navigation parameters. When one of the integrated electronics standby instruments is reconfigured to display flight parameters, the other instrument is automatically reconfigured to display navigation parameters, to avoid the simultaneous display of flight parameters that could be different. The automatic reconfiguration of one of the two integrated electronics standby instruments by the adjustment of the other is obtained by means of communication between the two integrated electronics instruments that use, for example, a serial link produced by means of an electrical cable linking the two integrated electronics standby instruments. The protocol for data transfer over the serial link uses, for example, a digital data transmission standard known by the standard name ARINC (Aeronautical Radio Incorporation, located at 2251 River Road, ANNAPOLIS, MARYLAND 21401 USA).
In the current situation using two integrated electronics standby instruments, in the event of failure of the sensors of a first integrated electronics standby instrument and failure of the display of the second integrated electronics standby instrument, there is no way left to display the flight parameters whereas the information concerning the flight parameters remains available.